As a band-pass filter used over a wide frequency range from a micro-wave band to a millimeter-wave band, there has been known a waveguide filter using a waveguide. The waveguide filter is low in loss and is high in power durability and is widely employed in a communication device mounted in an artificial satellite and a communication device in a ground station and the like.
As one type of the waveguide filter, JP-2006-121463-A discloses a waveguide filter in which a rectangular waveguide is divided into two parts at a center position of an H-plane thereof and along a direction in which an electromagnetic wave propagates in the waveguide by a plane vertical to the H-plane, that is, an E-plane, and in which a thin metal plate arranged parallel to the E-plane of the waveguide is sandwiched by the two divided parts and in which a dielectric plate is arranged on at least one portion above or below the metal plate. The dielectric plate is arranged so as to be parallel to the E-plane of the waveguide in such a way as to extend in a longitudinal extension direction of the metal plate. A dielectric material having a relative permittivity of 1.0 or more is used as the dielectric plate. The metal plate is designed in the shape of a ladder in such a way as to have a pass band at a specified frequency. In other words, the metal plate has windows periodically formed therein along the propagation direction of the electromagnetic wave. The loading of the dielectric plate produces an effect of changing an electric length in the direction of the H-plane. Thus, when the thickness and the arrangement position of the dielectric plate are changed, the center frequency of the waveguide filter can be changed, that is, a frequency shift of the center frequency can be achieved with a pass bandwidth held almost constant.
However, in the band-pass filter disclosed in JP-2006-121463-A, the shape of the metal plate or the dielectric plate constructs a coupling coefficient necessary for the band-pass filter, so that in order to change the center frequency, the metal plate or the dielectric plate that constructs the filter needs to be replaced by an other metal plate or dielectric plate. In other words, the band-pass filter disclosed in JP-2006-121463-A cannot be used as a tunable filter capable of changing the center frequency of a pass band during an operation.
In WO2006/075439 is disclosed a band-pass filter that uses a semi-coaxial cavity resonator and that can change a center frequency from the outside without putting a hand into the filter, or is tunable. In this tunable filter, an outer conductive part has a plurality of separate cavity portions formed therein, the cavity portions constructing respective stages of the resonator. The adjacent cavity portions are electromagnetically coupled to each other. The degree of electromagnetic coupling between the adjacent cavity portions can be adjusted by a coupling adjustment screw. Each of the cavity portions has an inner conductive part and a frequency adjustment screw provided therein as well as a dielectric member movably inserted therein, the dielectric member being fixed to a holding member. The holding member is projected to the outside of the outer conductive part. The outer conductive part has a link member provided on the outside thereof, the link member being common to the respective cavity portions. The holding member of each cavity portion is coupled to this link member. When the link member is slid or rotated, the distance between the dielectric member and the inner conductive part is changed in each cavity portion and hence the resonance frequency of each cavity portion as a resonator can be changed concurrently.
However, in order to realize excellent pass band characteristics in this tunable filter, the filter needs to be adjusted for each resonator of each stage by using the frequency adjustment screw and the coupling adjustment screw. Thus, in the case where the resonator has a large number of stages, this adjustment becomes extremely troublesome. Further, in order to realize a frequency shift, many parts such as the holding member, the link member, the dielectric member, and the drive unit need to be prepared.